Levels of soluble apolipoprotein E/amyloid-β (Aβ) complex are reduced and oligomeric Aβ increased with APOE4 and Alzheimer disease in a transgenic mouse model and human samples.

Department of Anatomy and Cell Biology, University of Illinois, Chicago, Illinois 60612, USA.

Abstract

Human apolipoprotein E (apoE) isoforms may differentially modulate amyloid-β (Aβ) levels. Evidence suggests physical interactions between apoE and Aβ are partially responsible for these functional effects. However, the apoE/Aβ complex is not a single static structure; rather, it is defined by detection methods. Thus, literature results are inconsistent and difficult to interpret. An ELISA was developed to measure soluble apoE/Aβ in a single, quantitative method and was used to address the hypothesis that reduced levels of soluble apoE/Aβ and an increase in soluble Aβ, specifically oligomeric Aβ (oAβ), are associated with APOE4 and AD. Previously, soluble Aβ42 and oAβ levels were greater with APOE4 compared with APOE2/APOE3 in hippocampal homogenates from EFAD transgenic mice (expressing five familial AD mutations and human apoE isoforms). In this study, soluble apoE/Aβ levels were lower in E4FAD mice compared with E2FAD and E3FAD mice, thus providing evidence that apoE/Aβ levels isoform-specifically modulate soluble oAβ clearance. Similar results were observed in soluble preparations of human cortical synaptosomes; apoE/Aβ levels were lower in AD patients compared with controls and lower with APOE4 in the AD cohort. In human CSF, apoE/Aβ levels were also lower in AD patients and with APOE4 in the AD cohort. Importantly, although total Aβ42 levels decreased in AD patients compared with controls, oAβ levels increased and were greater with APOE4 in the AD cohort. Overall, apoE isoform-specific formation of soluble apoE/Aβ modulates oAβ levels, suggesting a basis for APOE4-induced AD risk and a mechanistic approach to AD biomarkers.

ApoE/Aβ complex levels and stability in soluble brain extracts from EFAD mice.A, standardization and control for apoE/Aβ levels in the soluble (TBS) extraction fraction from the hippocampus and cerebellum of E3FAD mice at 6 months. B, apoE/Aβ complex in the soluble extraction fraction from the hippocampus of E2FAD, E3FAD, and E4FAD mice at 6 months. C, soluble apoE/Aβ stability in 0.02% and 0.2% SDS in samples as described for B. Standard curve for apoE/Aβ ELISA, 140.0 nm HEK-apoE3 with 0.15–50.0 nm unaggregated Aβ42. For all experiments n = 8 with duplicate samples. Data are expressed as mean ± S.E. and were analyzed by one-way ANOVA followed by Tukey's post hoc analysis (B), or by two-way ANOVA followed by Bonferroni post hoc analysis (C). *, p < 0.05.

AD prediction by Aβ42, oAβ, apoE/Aβ, T-Tau, and p-Tau-181 in human CSF using ROC curves. ROC curves for Aβ42, oAβ, apoE/Aβ, T-Tau, and p-Tau-181 in control and AD patients with the APOE3/3 genotype in human CSF are shown. ROC curves represent the predicted probabilities of being an AD case using marginal logistic regression models. Specificity (true negative rate, the proportion of AD controls correctly predicted) is plotted on the x axis and sensitivity (the proportion of AD cases correctly predicted) is plotted on the y axis, as calculated based on each subject's predicted case probability being above or below the varying threshold, respectively.